Nonalcoholic fatty liver disease (NAFLD) is a multifactorial clinicopathologic condition characterized by marked lipid accumulation in hepatocytes. The prevalence ranges from 10-24% across a spectrum of populations with upwards to 58% of obese adults and 53% of obese children affected by this disorder. The mechanisms underlying the transition of this disease to the more advanced stage, (NASH), characterized by hepatosteatosis, hepatocyte death and fibrosis remain to be elucidated although oxidative stress is proposed to play a central role in this progression. Oxidative stress is characterized by the production of reactive oxygen species (ROS) which initiate lipid peroxidation giving rise to bioactive aldehydic products of lipid peroxidation including 4-hydroxy-2-nonenal (4-HNE) and 4-oxononenal (4-ONE). It is our working hypothesis that specific protein modifications by 4-HNE and 4-ONE play important roles in the pathogenesis of NASH. This hypothesis will be tested by systematic experiments proposed in three specific aims using a mouse model of dietary-induced fatty liver. Experiments in Aim 1 will establish mechanistic relationships of documented biochemical and metabolic hallmarks of NAFLD and NASH with hepatic oxidative stress to delineate the ability of 4-HNE and 4-ONE to orchestrate the "second hit". Parameters to be characterized which emerge during the progression of NAFLD and NASH include hepatic histopathology, insulin sensitivity, dysregulation of lipid homoeostasis, increased production of TNF-a, decreases in adiponectin, the overproduction of proinflammatory cytokines as well as inflammation and fibrosis. Experiments in Aim 2 will identify hepatic proteins modified by 4-HNE and 4-ONE with the goal of identifying mechanistic links with initiation and progression of NASH. Experiments are proposed using the candidate proteins, long-chain acyl CoA dehydrogenase and the ER stress modulator GRP78 to evaluate the functional consequences of protein modification by 4-HNE and 4-ONE produced in association with NASH. Novel studies in Aim 3 will employ proteomic approaches to evaluate the hepatoprotective mechanisms of taurine supplementation in arresting NAFLD and NASH by restoration of ER stress response and protection against TNF-alpha-induced cell death. These proposed experiments will provide new insight into the pathomechanisms of NAFLD and potential therapeutic approaches to the treatment of this liver disease.